The present invention relates to a process for generating heat from combustion of a fuel containing sulphur, with reduced emissions of oxides of sulphur.
For a number of years, regulations limiting the emission of oxides of sulphur by the gaseous effluents from heat generators have been tightened; they forbid the use of fuels with high sulphur contents without subsequent treatment of the oxides of sulphur contained in the combustion fumes. The combustion of certain residues or mined materials, such as certain lignite coals, or petroleum residues and heavy fuels from refining processes, however, have a number of economic advantages. A variety of processes and/or apparatus described in the prior art can reduce the emissions of oxides of sulphur and more particularly sulphur dioxide from a heat generator by treating the combustion fumes before their discharge.
As an example, certain heat generators comprise a combustion chamber, at least one heat exchange zone, a desulphurisation chamber comprising at least one means for injecting a solid adsorbent, this chamber communicating with a convection exchange zone, then a separation chamber connected to said convection exchange zone (FIG. 1). The separation chamber comprises at least one outlet for used adsorbent and at least one outlet for purified gas. French patents FR-A-2 636 720 and FR-A-2 664 022 describe a process and apparatus for generating heat comprising desulphurising effluents with adsorbent particles with a fine particle size in a transported bed. The adsorbent injected into these heat generators essentially consists of a calcitic material that is sent, after partial sulphurisation, to a storage hopper (or final dust collector) generally constituted by a bag filter or an electrostatic filter. That type of generator results in the production of non negligible quantities of used calcitic adsorbent, i.e., sulphated, resulting in non negligible problems as regards storage and/or upgrading of that waste.
In order to limit the quantity of used adsorbent, an improvement to such a process consists of using xe2x80x9cregeneratablexe2x80x9d adsorbents essentially constituted by magnesian compounds (dolomite, magnesium oxide, magnesium carbonate) as claimed in FR-A-2 692 813.
FR-A-2 671 855 discloses an apparatus using such regeneratable adsorbents by adding an adsorbent regenerating means to the apparatus described above and means for treating the adsorbent and/or regeneration gases arriving from the regeneration means. Such a regeneration means can, for example, be a filter reactor such as that described in FR-A-2 730 424.
External recycling of the adsorbent causes a number of problems, however:
substantial movement of solid in the heat generator and as a result, substantial over-dimensioning of the apparatus assembly (in particular the size of the electrostatic filter and the heat exchangers in the convection exchange zone);
wear and clogging in the tubes, necessitating frequent cleaning operations;
a reduction in the overall energy yield of the heat generator linked to substantial heat losses of the adsorbent in the recycle circuit.
In parallel, the prior art, in particular FR-A-2 748 402, discloses a unit for treating the fumes from incinerating household refuse comprising an internal recycle. That apparatus, known as a CRI reactor, has been characterized and it has been shown that its performance is substantially superior to that of a traditional straight reactor, as regards fume dechlorination. The dechlorination apparatus comprises a peripheral zone for recycling adsorbent, an intermediate zone for desulphurisation and a central zone for evacuating purified fumes to an external convection exchange zone.
The aim of the present invention is to provide a process for integrating a CRI reactor, or any other reactor functioning using the same principal of internal recycling of the adsorbent, to applications other than the treatment of fumes downstream of a household refuse incinerator and to dechlorination. While the nature of the adsorbents, the reaction temperatures and the reaction mechanisms are different, it has been discovered that the CRI reactor surprisingly exhibits a number of advantages and unexpected effects in the field of desulphurising fumes when it is integrated (or juxtaposed, depending on the apparatus) with a heat generator in accordance with the process of the present invention, allowing the adsorbent to be recycled directly to the heat generator and not from the final dust collector:
the proposed solution can increase desulphurisation yields since, for an identical reaction volume, the fumes are in contact with a much larger quantity of adsorbent than in the traditional straight reactor as claimed in FR-A-2 636 720, FR-A-2 664 022 and FR-A-2 671 855;
regardless of the configuration of the heat generator, the temperature in the CRI reactor can be kept much more constant than with a straight reactor with an external recycle of the adsorbent from the final filter;
desulphurisation is less costly from the energy point of view, insofar as the recycled adsorbent is already at the reaction temperature, while in prior art configurations, the adsorbent is at a temperature close to ambient temperature and its injection into the desulphurisation apparatus is accompanied by substantial absorption of the heat transported by the fumes;
to function in accordance with the process of the present application, the CRI apparatus can be mounted on an existing installation, by means of some modifications in the fume circuit and the exchange surfaces in the boiler. This possibility did not exist in prior patented configurations. Thus, existing installations can be remodeled to integrate the CRI reactor using the present process, to bring them into line with the ever more stringent regulations regarding atmospheric discharges;
since the adsorbent is recirculated in the desulphurisation apparatus, the flow rates of the adsorbent that traverse the convection exchange zone are low; this means that substantial over-dimensioning, which is always necessary when large quantities of dust are present, can be avoided (for example it is possible to envisage a smaller space between the tubes in the heat exchanger), and it can also limit clogging and wear in the tubes, further limiting cleaning operations;
a smaller amount of adsorbent is manipulated outside the generator, limiting the size and thus the cost of apparatus for handling solids;
the solids arriving at the final dust collector of the heat generator (bag filter or electrostatic filter, for example) are only constituted by adsorbent to be regenerated in the case of regeneratable adsorbent or adsorbent to be evacuated in the case of non regeneratable adsorbents, and not a large quantity of adsorbent to be recycled, so the dimensions of the final dust collector can be substantially reduced, saving space and costs.
Clearly, the invention is not limited to the use in the present process of the CRI reactor described in FR-A-2 671 855; the present invention also encompasses using any type of reactor with the same internal adsorbent recycle properties.
In general, the invention concerns a heat generation process with reduced emissions of oxides of sulphur in which:
a) a sulphur-containing fuel is burned in a combustion zone comprising a heat exchange zone in which at least a portion of the heat is extracted, and effluents or combustion fumes are recovered at a temperature in the range 800xc2x0 C. to 1200xc2x0 C.;
b) the fumes resulting from said combustion, charged with oxides of sulphur, are caused to traverse a space for supplying and distributing fumes in a desulphurisation apparatus functioning with an internal recycle of a solid oxides of sulphur adsorbent;
c) the adsorbent is injected into said space;
d) the fumes are caused to enter said apparatus;
e) the fumes are caused to penetrate into a convection exchange zone and at least a portion of the heat is extracted from said fumes;
f) the mixture resulting from steps b) and c) is separated in a gas/solid separation zone and a portion of the gaseous effluent that has been freed of the major portion of the oxides of sulphur and at least partially cooled is evacuated, and said adsorbent particles comprising said sulphur-containing compounds are evacuated.
In one embodiment, the desulphurisation apparatus comprises a peripheral zone for recycling adsorbent, an intermediate zone for desulphurisation into which fumes enter tangentially, and a central zone for evacuating fumes.
Advantageously, after step f), at least a portion of said adsorbent particles comprising said sulphur-containing compounds is regenerated and the regenerated adsorbent particles are re-injected into the space acting to supply the desulphurisation apparatus.
It is possible to use a calcitic adsorbent in which case the mean desulphurisation temperature is in the range 800xc2x0 C. to 110xc2x0 C.
It is also possible to use a regeneratable magnesian adsorbent, in which case the mean desulphurisation temperature is in the range 700xc2x0 C. to 1000xc2x0 C.
Preferably, after combustion step a), the fumes traverse one or more superheated steam banks. In general, the adsorbent flow rates are such that the concentration of solids in the fumes, except for the recycle, is in the range 0.1 to 1000 g/Nm3, the gas recycle ratio in the apparatus is in the range 1% to 50%, and the adsorbent recycle ratio is in the range 1 to 50.
Preferably, the grain size of the adsorbents is in the range 0.1 to 1000 microns, and the density of the adsorbent particles is in the range 100 to 5000 kg/m3.